Summary

The current distribution of C3 and C4 photosynthetic pathways in today's ecosystems is a strong function of temperature. Changing atmospheric C02 levels modify this geographical distribution. The global emergence of C4-dominated ecosystems in the late Miocene suggests that atmospheric C02 levels decreased across a threshold of ~ 500 ppmV favoring C4 photosynthesis over C3 photosynthesis in warm ecosystems. More recently during glacial periods when atmospheric C02 levels decreased to 180 ppmV, C4 taxa were apparently more abundant than they are today. These changes in C3/C4 abundances have had enormous impacts on both evolution and distribution of mammalian grazers. The mechanistic basis for this impact on mammal herbivory may be feeding preferences associated with differential digestibility of C3 versus C4 grasses.

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